1. Field of the Invention
The present invention relates to a second harmonic wave generating device useful as a light source for example for optical information recording and regeneration, or for semiconductor manufacturing processes.
2. Related Background Art
Semiconductor lasers have been generally employed for such light sources. However it is difficult to utilize the semiconductor laser as a light source of a short wavelength, because it can only emit photons of an energy close to the band gap energy Eg determined by the material constituting the active layer. For this reason there has been proposed a device for obtaining light of a short wavelength, utilizing second harmonic wave generation, as schematically illustrated in FIG. 1.
In FIG. 1, a semiconductor laser 66 emits light 40 of a wavelength .lambda. when a current 64 is injected. Said light 40 enters a second harmonic generating (SHG) material 67 with a large second nonlinear constant .chi., which generates a second harmonic wave 42 of a wavelength .lambda./2 together with a transmitted wave 41 of the wavelength .lambda..
FIGS. 2A and 2B illustrate details of a conventional second harmonic generating device, wherein FIG. 2B is an elevation view seen from the proceeding direction of the light, while FIG. 2A is a schematic cross-sectional view along a line B--B in FIG. 2B. Said device comprises a basic wave generating light source 60 and a converter element 61 for converting the wavelength of the basic laser beam.
When the basic wave (angular frequency .omega.; wavelength .lambda..sub.0) is introduced from the light source 60 to a wave guide channel 62 of the converter element 61, a part of said basic wave propagates in a diagonally downward area 63 with respect to said wave guide channel 62 and is converted into a second harmonic wave (angular frequency 2.omega.; wavelength .lambda..sub.0 /2). The propagating area 63 for the second harmonic wave is diagonally downwards because the second harmonic wave matches in phase with the basic wave in this direction.
A converter element 61 composed of a LiNbO.sub.3 crystal in which the wave guide channel 62 is formed by replacing Li.sup.+ in said crystal with H.sup.+ by ion exchange is described in "SHG element utilizing ridge-type LiNO.sub.3 wave guide channel" Preprints for 48th Symposium of Academy of Applied Physics, 19P/ZG/3, Fall 1987. According to this publication, this converter element emits a second harmonic wave of a wavelength of 0.42 .mu.m upon receiving a semiconductor laser beam of a wavelength of 0.84 .mu.m as the basic wave.
Such second harmonic wave generating device, if utilized in the recording or regeneration of an optical recording medium such as an optical disk, can reduce the area of the light spot by 1/4 by reducing the wavelength of the laser beam by 1/2, thereby quadrupling the optical recording density. It can also be utilized in semiconductor manufacturing processes utilizing light, such as exposure, etching or doping, for increasing the pattern density.
However the above-explained device has been associated with the following drawbacks that:
(1) it is difficult to efficiently introduce the basic wave into the converter element, due to the reflection at the entrance end face thereof; PA1 (2) the entire device is bulky since the converter element is inevitably several centimeters long, though the semiconductor laser itself for generating the basic wave is less than 1 mm in size; and PA1 (3) the conversion efficiency is low, for example about 2.5% in the above-mentioned LiNbO.sub.3 crystal as only a small portion of the basic wave is converted into the second harmonic wave. PA1 a substrate; PA1 an active layer formed on said substrate and adapted for generating light of a wavelength .lambda. in response to the injection of a current; PA1 a second harmonic wave generating layer formed on said substrate and adapted to generate light of a wavelength of .lambda./2 from the light generated in said active layer; and PA1 a pair of electrodes for supplying said active layer with a current.